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Bitumen Cutback Grades
Bitumen Cutback grade asphalt is a versatile material used in the construction and maintenance of roads. It is created by blending asphalt cement with petroleum solvents, which are known as distillate, diluents, or cutter stock. The solvents used in cutback asphalt play a crucial role in determining its properties, and based on the relative speed of evaporation, cutback grades are classified into three types.
• Rapid Curing (RC)
• Medium Curing (MC)
• Slow Curing (SC)
Rapid Curing (RC) Cutback Bitumen:
Rapid Curing (RC) cutback asphalt is a combination of light diluents with high volatility, typically in the gasoline or naphtha boiling point range, and asphalt cement. The addition of kerosene, usually between 28% and 33%, further influences the viscosity of the asphalt. As the level of kerosene increases, the viscosity of the asphalt decreases. RC cutback grades, such as RC-30, RC-70, RC-250, RC-800, and RC-3000, are designed for various applications.
RC-70, for example, is applied for the waterproofing of surfaces, helping to plug capillary voids and coat and bond loose mineral particles. The fluidity of RC grades depends on the ratio of solvent to asphalt cement, and the more viscous grades may require heating for construction operations. Rapid-curing grades are primarily used in spray applications like bond/tack coats, aggregate chip seals, sand seals, granular priming, and similar surface treatments.
Medium Curing (MC) Cutback Bitumen:
Medium Curing (MC) cutback asphalt, like MC-30, MC-70, MC-250, MC-800, and MC-3000, is a combination of asphalt cement and petroleum solvent. Similar to emulsified asphalts, cutbacks are used to reduce asphalt viscosity for lower-temperature applications such as tack coats, fog seals, slurry seals, and stabilization materials. After application, the petroleum solvent evaporates, leaving behind asphalt cement residue on the surface.
However, the use of cutback asphalts is decreasing due to environmental regulations and the loss of high-energy products associated with the manufacture of petroleum solvents. Slow Curing (SC) cutback bitumen and oils of low volatility, such as SC-70, SC-250, SC-800, and SC-3000, are part of this category. The degree of liquidity developed in each case depends on the proportion of solvent to asphalt cement.
Slow Curing (SC) Cutback Bitumen:
Slow Curing (SC) cutback bitumen, often referred to as “Road Oil”, consists of bitumen blended with oils of low volatility. These oils are generally in the heavy distillate range, and the resulting material may require a small amount of heating for construction operations. Slow Curing grades are suitable for applications like penetrating prime coats and producing patching or stockpile mixtures.
Slow cutback grade (SC) bitumen is applied for waterproofing surfaces, plugging capillary voids, and coating and bonding loose mineral particles. The incorporation of an adhesion agent in the mixture assists in the coating of the aggregate surface. Kerosene is commonly used as a cutback agent in different concentrations based on local conditions and requirements. The use of cutback asphalts is decreasing because of:
- Environmental regulations: Cutback asphalts contain volatile chemicals that evaporate into the atmosphere. Emulsified asphalts evaporate water into the atmosphere.
- Loss of high energy products: The petroleum solvents used require higher amounts of energy to manufacture and are expensive compared to the water and emulsifying agents used in emulsified asphalts.
- Slow Curing Cutback grade Bitumen and oils of low volatility generally in the heavy distillate range (SC–70, SC–250, SC–800 & SC–3000). The degree of liquidity developed in each case depends principally on the proportion of solvent to asphalt cement. To a minor degree, the liquidity of the cutback may be affected by the hardness of the base asphalt from which the cutback is made. The degree of fluidity results in several grades of cutback asphalt–some quite fluid at ordinary temperatures and others somewhat more viscous. The more viscous grades may require a small amount of heating to make them fluid enough for construction operations. Slow Curing often called “Road Oil” is usually a residual material produced from the fractional distillation of certain crude petroleum. Traditionally any kind of aromatic, naphthenic and paraffinic oils are used. Slow Curing liquid bitumen materials can be prepared by blending bitumen with an oily petroleum fraction.
In summary, cutback grade asphalt grades play a crucial role in various road construction and maintenance applications. Whether it’s the rapid-curing RC grades for quick-setting applications or the slower-curing SC grades for specialized purposes, the choice of cutback asphalt depends on the specific requirements of the project and local conditions. As environmental considerations continue to shape the industry, the use of cutback asphalts may evolve, but their versatility in road construction remains significant.
